NUMERICAL SIMULATIONS OF CONVECTION AT THE SURFACE OF A ZZ CETI WHITE-DWARF

We applied two-dimensional hydrodynamics and non-grey radiative transf er calculations to the surface layers of a hydrogen-rich white dwarf ( spectral type DA) with T(eff) = 12600 K and log g = 8.0, corresponding to a position in the HR-diagram slightly cooler than the hot boundary of the ZZ Ceti instability strip. In our simulations the entire conve ction zone including the overshoot layers is embedded in the computati onal box so that we obtain a complete and detailed model of convection for this representative object. We address the important question to what extent models based on mixing length theory (MLT) are able to pre dict the physical properties of convection. We find a rapidly (timesca le almost-equal-to 100 ms) evolving flow pattern with fast concentrate d downdrafts surrounded by slow broad upflows of warmer material. Conv ection carries up to 30% of the total flux and excites internal gravit y waves by dynamical processes associated with the merging of down-dra fts. The mean entropy gradient is reversed with respect to MLT predict ions in the deeper layers of the convection zone. Strong overshoot occ urs at its upper and lower boundary. A synthetic spectrum calculated f rom the mean photospheric temperature stratification can be fitted sat isfactorily with a MLT model adopting alpha = 1.5. At greater depth th e temperature profile approaches a model with alpha = 4. The total dep th of the convective layers is rather small compared to values suggest ed by studies of the excitation mechanism for the pulsations of DAs.